Predicted three-dimensional structures of proEcCLP1 and mature EcCLP1 and comparison with the human cathepsin. (A) Topological representation of the mature EcCLP1 with the residues forming the S2 subsite marked in red. (B) proEcCLP1 showing the propeptide in cyan and blue. (C) Magnified image of the model shown in (A) showing the residues forming the S2 subsite in white with papain numbering in parentheses. (D) Superposition of the ribbon representation of the crystal structure of human cathepsin (brown) [6] and the modeled EcCLP1 (green). (E) Magnified image of the active site of the cathepsins shown in (D). Important S2 residues are marked in green in parasite numbering and red in human cathepsin. Differences between both sequences are marked with boxes.